All cells are separated from their environments by their cell membranes.  The cell membrane is a phospholipid protein bilayer which is a very good insulator.  The dielectric constant of the cell membrane is approximately 9.  The membrane separates the intracellular and extracellular fluids, which are conductors.   The free charges in these conductors are ions.  The interior of a cell is an equipotential volume and its potential is approximately -60 to -90 mV below that of the extracellular fluids for human cells.

The fluid both inside and outside of a cell contains a high concentration of dissolved salts.  When salts dissolve in water, they form positively and negatively charged ions.  In most cells, there is a higher concentration of potassium ions (K⁺) inside than outside the cell and a higher concentration of sodium ions (Na⁺) outside than inside the cell.  While the cell membrane is a good insulator, it contains channels through which ions can leak.  In most cases an ion channel is only permeable to specific types of ions.  Channels can be passive or active, and they can be always open or gated.  The presence of always open, passive potassium channels makes the cell membrane fairly permeable to potassium ions, and, because of the concentration difference, more potassium ions diffuse out of the cell than into the cell, leaving the interior of the cell negatively charged.  The electric field produced by the separated charges points towards the inside of the cell and accelerates K⁺ ions entering the cell while decelerating K⁺ ions leaving the interior.  This equalized the rates and establishes a resting potential difference between the outside and the inside of the cell.  The cell is polarized.  The cell membrane now acts like a capacitor, storing charge.  (The above description is a simplification.  Active channels or ion pumps are responsible for several millivolts of the resting potential.  Active channels pump ions against concentration differences and require energy input.)

Gated channels open when appropriate stimuli are present.  When gated channels open, the potential difference across the cell membrane changes.  The cell "depolarizes", and then "repolarizes" again when the gated channels close.  Electrical currents are generated.  Because the body is a good conductor, electrical activity can be monitored on the surface of the body.

For example, as the heart undergoes depolarization and repolarization, the electrical currents that are generated spread not only within the heart, but also throughout the body. This electrical activity generated by the heart can be measured by an array of electrodes placed on the body surface. The recorded tracing is called an electrocardiogram (ECG or EKG).